Hi!
I made a deb package with these addons http://supertuxkart.sourceforge.net/Downloads#Add-ons
(9 extra karts and 7 extra tracks)
If you want test it and upload it to be more easy to install!
I was tested with Ubuntu 10.04 & supertuxkart 0.6.2
Download for test here http://www.mediafire.com/?cc67ql5ag1zp6xw